1. Technical Field of the Invention
This invention relates to recovering precious metal values from refractory ores, ore concentrates, or ore tailings which include arsenic-, carbon- and sulfur-containing components and which are refractory to the recovery of those precious metal values.
2. Background Art
Precious metals, such as gold, occur naturally in ores in different forms. Unfortunately, precious metal ores also frequently contain other materials which interfere with the recovery of these precious metal values, rendering these ores refractory to precious metal recovery. Furthermore, the precious metal content may be at a relatively low level. This low level content compounds the effect of the refractory nature of these ores.
The following patents are illustrative of attempts to deal with refractory components in precious metals recovery as well as efforts in distinctly different fields.
U.S. Pat. No. 360,904 to Elizabeth B. Parnell relates to roasting gold or silver bearing ores using a double roasting schedule with the first roasting at 1100 to 1300 degrees Fahrenheit and the second roasting at 1200 to 1600 degrees Fahrenheit (the time occupied in the second roasting can be reduced by supplying oxygen along with the air.)
U.S. Pat. No. 921,645 to J.E. Greenwalt discloses the roasting of ore by heating the ore on a porous granular bed through which air is forced from below.
U.S. Pat. No. 1,075,011 to N.C. Christensen, Jr. discloses a process for treating ore by means of a roasting oven which, by regulation of the fuel supply, may be either oxidizing, reducing, or neutral.
U.S. Pat. No. 2,056,564 to Bernard M. Carter discloses suspension roasting of finely divided sulfide ores. Roasting is in air or oxygen in which the temperature of the mixture entering the roasting chamber is controlled and to a corresponding degree the temperatures within the roasting chamber are thus controlled in an effort to prevent the formation of accretions on the walls of the apparatus.
U.S. Pat. No. 2,209,331 to Ture Robert Haglund discloses a process for the production of sulfur from the roasting of sulfide material in oxygen or air enriched with oxygen so that as soon as the free oxygen has been consumed in the formation of SO.sub.2, the iron sulfide reacts with the sulfur dioxide forming free sulfur and iron oxides.
U.S. Pat. No. 2,536,952 to Kenneth D. McCean relates to roasting mineral sulfides in gaseous suspension.
U.S. Pat. No. 2,596,580 to James B. McKay et al. and U.S. Pat. No. 2,650, 169 to Donald T. Tarr, Jr. et al., relates to roasting gold-bearing ores which contain commercially significant amounts of gold in association with the mineral arsenopyrite. The patent describes the importance of closely regulating the availability of oxygen in order to provide enough oxygen so that volatile compounds of arsenic are formed while the formation of nonvolatile arsenic compounds is minimized.
U.S. Pat. No. 2,867,529 to Frank A. Forward relates to treatment of refractory ores and concentrates which contain at least one precious metal, sulfur and at least one arsenic, antimony or lead compound by roasting in a non-oxidizing atmosphere at a temperature above 900 degrees Fahrenheit, but less than the fusion temperature of the material being roasted.
U.S. Pat. No. 2,927,017 to Orrin F. Marvin relates to a method for refining metals, including precious metals, from complex ores which contain two or more metal values in chemical union or in such physical union as to prevent normal mechanical separation of the values. The method uses multiple roasting steps.
U.S. Pat. No. 2,993,778 to Adolf Johannsen et al. relates to roasting a sulfur mineral with its objects being-the production of sulfur dioxide, increasing the completeness of roasting, the production of sulfur dioxide and the production of metal oxides.
U.S. Pat. No. 3,172,755 to Angel Vian-Ortuno et al. relates to a process for treating pyrite ores bearing arsenic by subjecting the arsenic-containing pyrite ore to partial oxidation so as to oxidize only the labile sulfur of the arsenic-containing pyrite and subsequently heating the pyrite ore in a nonoxidizing gas to separate the arsenic from the ore and to form a residual ore free of arsenic.
U.S. Pat. No. 4,731,114 Gopalan Ramadorai et al. relates to a process for the recovery of precious metals from low-grade carbonaceous sulfide ores using partial roasting of the ores following by aqueous oxidation in an autoclave.
U.S. Pat. No. 4,919,715 relates to the use of pure oxygen in roasting of refractory gold-bearing ores at temperatures between about 1000.degree. F. (537.8.degree. C.) and about 1200.degree. F. (648.9.degree. C.). It fails to address the problem of arsenic volatilization or of optimizing gold recovery from refractory sulfidic, carbonaceous or separation of cyanide consuming components before recovery of gold from the ore. The disclosed method requires two stage roasting and the use of substantially pure oxygen (substantially pure oxygen being defined as at least about 80% by weight.)
None of these patents teaches o suggests roasting refractory ores, ore concentrates or ore tailings of the type described herein in an oxygen-enriched gaseous environment as described here in order to minimize and/or eliminate arsenic volatilization, facilitate arsenic conversion to an insoluble, environmentally acceptable form while reducing the effects of carbon- and sulfur-containing components on precious metal recovery. The present invention achieves these results in a simpler more efficient manner with outstanding gold recovery results while minimizing leachant cyanide consumption and conserving heat given-off in the roasting process.